reSmush.it Image Optimizer https://wordpress.org/plugins/resmushit-image-optimizer/
WP Smush.it: https://wordpress.org/plugins/wp-smushit/
ShortPixel Image Optimizer: https://wordpress.org/plugins/shortpixel-image-optimiser/
You can also use EWWW Image Optimizer, CW Image Optimizer, Tiny Png, Imsanity and Hammy.